The invention relates to the repair lacquering of defects within stoved coating layers using powder coating compositions.
Lacquer defects, such as, for example, craters, dents, scratches or inclusions of dirt, within stoving coating layers, for example within stoved coating layers produced from powder coatings, can be repaired using liquid lacquers. The repair process is costly in terms of time and is labour-intensive. For example, curing of the repair lacquer requires the action of elevated temperatures for a sufficiently long period of time. If the substrates to be repair lacquered are temperature-sensitive substrates, then the object temperature cannot be chosen as high as desired. In the case of substrates to be repair lacquered that are composed of a mixture of temperature-sensitive and non-temperature-sensitive components, the temperature-sensitive components are generally removed prior to the action of heat, for example in a stoving oven, and are subsequently re-fitted. Such measures hinder series lacquering processes in particular, for example the lacquering of motor vehicles with filler, finishing or clear lacquers, and render them more expensive.
A particular problem in the repair lacquering, using liquid lacquers, of external, visible stoving finishing lacquers produced from powder coatings is that of carrying out the repair lacquering in such a manner that the repaired area is not visually conspicuous, for example in respect of colour matching or gloss. In addition, in the case of the repair of stoved clear powder coating layers, the refractive index of the cured clear repair lacquer must not differ substantially from the refractive index of the stoved clear powder coating.
Methods of repairing defective areas in lacquer layers while avoiding conventional thermal curing by convection or conventional IR irradiation are known from DE-A-38 33 225 and DE-A-197 20 894. The methods described therein use laser light as the energy source for curing of the repair lacquer. DE-A-197 20 946 describes a method for repairing defective areas in a stoving lacquer coating, wherein the defective area ready for repair is coated with a lacquer coating composition and the applied lacquer is then cured by irradiation with a laser beam source in the form of a Nd:YAG laser, which produces near infrared radiation.
From EP-A-0 842 710 there is known a method of repairing defective areas in powder coating layers in which a filling body is introduced into the defective area which has been prepared for repair, for example ground or milled out, and is bonded therewith. The filling body preferably corresponds to the dimensions of the prepared defective area. EP-A-0 887 118 improves the method known from EP-A-0 842 710 in respect of the achievable quality of the repaired defective area, by carrying out the method known from EP-A-0 842 710 in such a manner that the bonding of the filling body and the defective area takes place under the action of pressure. Although the methods of EP-A-0 842 710 and EP-A-0 887 118 avoid the disadvantages of repairing defective areas in powder coating layers by means of liquid lacquers, they are nevertheless complex because of the necessary preparation of the filling body, especially in the dimensions adapted to the defective area to be repaired.
In the article “Sekundenschnelle Aushärtung von Pulverlack” [Curing powder coatings in seconds] (Kai Bär, JOT 2/98, pages 26 to 29), it is described that powder coatings can be cured with the aid of high-intensity near infrared radiation (NIR) without any substantial heating of the substrate occurring. NIR technology allows powder coatings to be melted and cured in a single process step.
The object of the invention is to provide an improved method for the repair lacquering of defective areas in stoving lacquer coatings, especially within stoving lacquer coatings produced using powder coatings, which method avoids the described disadvantages of the prior art. The method should in particular also be suitable for the repair lacquering of series-lacquered, industrially manufactured articles, especially motor vehicles and parts thereof, for example within the framework of or subsequent to series lacquering.